This invention relates to improvements in methods and apparatus for measuring the flow of fluid in a duct. In particular, although not exclusively, it relates to insertion monitoring, in which a flow measurement probe is inserted through an opening in a wall of a duct, such as a pipe.
It is becoming increasingly common place to install fixed flow rate monitoring equipment into a duct network. An example is the provision of water meters or gas meters in the domestic utility companies pipe networks. By providing a separate meter for each household, the flow of fluid and hence volume of fluid used over time by each household can be calculated.
Because fixed meters can by their very nature not be readily removed without considerable difficulty, it has been found that the only reliable way to test the integrity and calibration of the measuring devices in the meter is to take independent on-line measurements of flow rate. These measurements can then be compared with the readings from the fixed meters. Typically, the on-line measurements are obtained by using data from insertion flow measurement devices which are inserted through an opening in the wall of the duct. In some applications, the insertion flow measuring devices are removed following testing. However, it is also possible to leave the device in the duct for a longer period, say 12 or 24 hours, or perhaps permanently.
At present, the use of the insertion monitoring techniques that exist in the state of the art prior to the present invention is restricted by financial and practical problems. In one known insertion metering device, a probe is inserted into the duct through a hole or valve opening in the duct wall. The probe comprises a rod which carries a turbine or electromagnetic sensing element on its tip. The sensing element can take a point measurement indicative of the flow in a part of the pipe at a point in time. However, because the flow in the pipe is unknown, (varying both in profile across the cross-section of the pipe and with time) several measurements must be taken at different points in the cross-section of the duct and at different times. An average can then be built up which would approximate the average flow rate. Its accuracy is limited by the difficulty in aligning the sensing element correctly along the axis of the duct.
In order to obtain reasonably accurate results, the prior art insertion technique requires that measurements are taken at several positions across at least one diameter of the pipe. However, it has been found that in practice where flow profiles are distorted that it is necessary to measure across more than one diameter (i.e. two orthogonal diameters) to provide sufficiently accurate results which can be used for calibration. This introduces severe problems when the duct system is installed underground, as it requires that a large chamber must be excavated around the pipe in order to allow access for separate circumferentially spaced holes in the pipe to be made to allow the orthogonal measurements to be made. To make the chambers can be both expensive and time consuming.
A further problem with the prior art technique is that the surface area of the rod which supports the sensing element forms a variable blockage in the duct as the element is moved across the diameter. This blockage affects the results by altering the flow profile in the duct and increases turbulence. Furthermore, the process of taking the many measurements required is subject to variability due to the often difficult operating conditions in which the measurements must be made. Often, the insertion probe operator may be working in a water filled, muddy pit which makes it difficult to obtain the various readings with any certain degree of accuracy. Different operators can get different results. It is thus desirable to de-skill the measurement process.
According to a first aspect of the invention we provide an ultrasonic insertion flow meter having a probe adapted to be inserted into a duct, said ultrasonic probe having ultrasound transducers and being adapted to perform at a single site of introduction into the duct a first ultrasonic path interrogation having a component of travel of ultrasound in a first direction that is, in use, an axial direction relative to the region of the duct where the device is inserted, and also adapted to perform a second ultrasound path interrogation having a component of travel of ultrasound in a second axial direction opposite to said first axial direction, the arrangement being such that a comparison of the signal associated with ultrasound travel in one axial direction with that of the signal associated with ultrasound travel in the opposite axial direction enables the flow rate of fluid in said duct to be estimated.
Time of flight measurements of ultrasound are effected by the flow rate and the insertion meter uses a transit time ultrasonic measurement to evaluate the flow rate.
The meter has an ultrasound emitter and an ultrasound detector.
These may be different transducers, but we prefer to use the same transducer to emit and detect. It may be possible to use appropriate reflectors spaced in use axially (relative to the duct) of a combined emitter/receiver transducer.
There may be a first emitter and receiver pair spaced apart by spacing means. There may be a second emitter and receiver pair, which may be spaced apart by the same spacing means.
Preferably the probe is adapted to measure the transit time difference of an ultrasonic pulse in the forward and reverse direction of a first interrogation path and also the transit time difference in the forward and reverse directions of a second, different, interrogation path. The average of the transit time difference in forward and reverse directions of ultrasound travel along the different interrogation paths gives a representation of the flow rate of fluid in the duct. The difference between the forward and reverse transit times of the different interrogation paths may be indicative of the swirl of fluid in the duct.
Because the ultrasound path has at least two path environments and because the ultrasound does travel through the fluid in the pipe axially (at last with an axial component) rather than a single point measurement of flow being obtained as in the prior art, the fluid flow at man different points on the ultrasound path effects the signal that i s measured. This provides a degree of in-built averaging or integration which eliminates the need to obtain many measurements at different points in the cross-section of the duct. This in turn means that the measurement process is quicker and requires less skill.
A first pair of ultrasonic transducers may be provided on one transducer mounting and may be spaced from a second pair of ultrasonic transducers provided on a second transducer mounting. In this case, the first and second pairs of transducers may communicate along two different geometric paths. The first pair of transducers may comprise the emitter of one emitter/receiver pair and the receiver of another emitter/receiver pair. The second pair of transducers may comprise the receiver of said one emitter/receiver pair and the transmitter of said emitter/receiver pair.
Preferably, the insertion meter is adapted to use the reflection of ultrasound off the sidewalls of the duct to create the first ultrasonic path interrogation and/or the second ultrasonic path interrogation. The ultrasonic paths may be beams of ultrasound. There may be more than one reflection off the duct walls as the ultrasound travels from its emitter to its receiver.
The insertion flow meter probe is adapted to take the first and second ultrasonic path interrogations whilst it is stationary.
The first emitter/receiver pair is preferably adapted to be spaced axially of the duct in use, preferably in a direction parallel to the central longitudinal axis of the duct. Similarly, the second emitter/receiver pair is preferably adapted to be spaced axially of the duct in use, preferably in a direction that is parallel to the axis of the duct.
Preferably the or each emitter is adapted to be adjacent a wall of the duct in use. Preferably the or each receiver is adapted to be adjacent a wall of the duct, most preferably the same wall as the emitter(s).
The emitter/receiver of a first pair may be spaced emitter-to-receiver in one axial direction and the emitter/receiver of the second pair may be spaced emitter-to-receiver in the opposite direction.
Two beams of ultrasound may be used to interrogate the flow in the duct, one beam when viewed along the axis of the duct having reflections having a clockwise sense of rotation about the axis, and the other beam when viewed along the same direction having reflections in an anticlockwise sense of rotation about the axis. This enables a comparison of the two signals to have a reduced or eliminated sensitivity to swirl of fluid in the duct.
Preferably the flow meter comprises an insertion means adapted to support the probe means and adapted to move the probe means in use relative to the duct between a first position in which said probe means can be inserted into the duct and a second position in which flow measurements can be made.
The probe means preferably has a greater dimension parallel to the axis of the duct in its second position than it does in its first position.
Preferably, in the first position, the probe means is oriented so that the first and second transducers are spaced apart substantially orthogonal to the axis of the duct. In the second position the probe means is preferably oriented so that the pairs of transducers are spaced apart substantially axially along the duct. This is advantageous as it allows for the orifice in the duct through which the probe means is inserted into the duct to be of a small size. After insertion, the probe means can then be moved to take up its measurement position. The probe means is preferably swung from its introduction position to its measurement position.
The spacing means between the first and second emitter/receiver pairs of transducers may be varied. Varying the spacing between the first and second transducers is advantageous as it allows the probe to be tailored for use with ducts of varying diameters (i.e. a large spacing could be used for measurements in large diameter ducts).
Preferably the probe means is hingedly attached to the insertion means so that the probe means can be moved angularly between the first position and the second position. The probe means may be hinged at one end. Alternatively, it may be hinged at any point along its length (e.g. the middle) so that, say, one of the pairs of transducers lies on either side of the hinge.
The flow measuring apparatus may further comprise signal processing means adapted to process output signals from the emitters and receivers and to monitor the transit time for ultrasonic signals passing along two different paths.
Most preferably, the signal processing means is adapted to compare the transit times of signals in the forward and reverse directions along the duct, i.e. signals from the first emitter/receiver pair and signals from the second emitter/receiver pair along each of the two paths.
Most preferably, the transducers are arranged so that in the second position of the probe means, the signals along each path are reflected twice from the wall of the duct, so that, when viewed along the axis of the duct, an angle of about 60 degrees is subtended between the incident and reflected signals at the point of reflection. This geometry has been found to be advantageous because it reduces the effect of the flow profile in the duct on flow measurements.
As previously mentioned, it is possible to provide a flow measuring apparatus which is adapted to compare the transit time for ultrasonic signals propagating in both directions along each of the two different paths. This comparison can be used to provide information about the amount of swirl of the flowing fluid as each path may be differently affected by the same amount of swirl.
In an alternative arrangement, the probe means may comprise first and second transducers which are mounted back to back between a pair of conic reflectors which are adapted so that the reflectors reflect signals from the first transducer to the second transducer and vice versa. The spacing of the reflectors relative to the transducers may be altered to allow the probe to operate with different duct sizes. Alternatively, the spacing between the reflectors and the transducer may be fixed, whilst the spacing between the first and second transducers could be varied.
According to another aspect of the invention we provide a method of measuring the flow rate of a fluid in a duct comprising the steps of:
inserting an insertion flow meter into a duct;
conducting a forward direction ultrasonic transit time test with a component of travel of the ultrasound in the axial direction of flow of the fluid in the duct;
conducting a reverse direction ultrasonic transit time test with a component of travel of the ultrasound in the direction opposite to the axial direction of flow of fluid in the duct;
comparing the results of the forward and reverse transit time tests to give a result indicative of the fluid flow rate.
Preferably the forward and reverse tests are performed between the same pair of transducers which act as emitter/receiver for the forward test and receiver/emitter for the reverse test.
Preferably a second forward direction test and/or a second reverse direction test is/are performed along a different ultrasound travel path than the first forward direction and reverse direction tests, and the result(s) from the second test(s) compared with those of the first travel path test to give a figure indicative of swirl of the fluid in the duct.
Preferably the method comprises performing the test with a probe that is in the same place for all of the tests.
Preferably the method comprises holding the probe to a wall of the duct, preferably the upper wall.
Preferably the method comprises orientating the probe in the duct by looking at the signals received and adjusting the orientation until a desired signal or comparison of signals is received.
Preferably the method comprises introducing the probe into the duct with the probe having a smaller projected length in the axial direction to the duct in comparison to its axial length in a position of use to which is manipulated prior to the tests being performed. The probe may be introduced generally radially to the duct used then moved to extend generally axially.
The method may comprise using an ultrasonic probe comprising at least one first ultrasonic transducer and at least one second ultrasonic transducer spaced apart from the first transducer, the first and second transducers being in a first position in which they are spaced apart substantially orthogonal to the duct axis when they are introduced into the duct,
moving the probe assembly from the first position to a second position in which the first and second transducers are spaced apart substantially along an axis of the duct;
measuring the transit time for pulses of ultrasound transmitted between the first and second transducers and vice versa along one or more paths; and
comparing the transit time for pulses propagating between the first and the second transducers to the transit time for pulses propagating between the second and the first transducers, the difference being indicative of the rate of flow of fluid in the duct.
Preferably, the probe assembly is rotatable between the first and second positions. Preferably, a pair of first transducers and a pair of second transducers is provided. Most preferably, the first and second pairs of transducers communicate along two different paths.
The method may further comprise the additional step of moving the probe assembly into contact with the upper surface of the duct, i.e. the surface adjacent the entry hole via which the probe is inserted. This is advantageous in that it reduces the area of the probe assembly presented to the fluid.
An advantageous feature of the invention is that by varying the alignment of the probe slightly relative to the axis of the duct and measuring the change in transit time of the signals produced by the transducers, it is possible to fine tune the position of the probe assembly so that it is correctly aligned with the axis of the duct. This feature, in addition to the self alignment obtained when the probe is pulled up against the duct wall allows excellent alignment of the probe relative to the axis to be obtained.
The method may further comprise the steps of adjusting the spacing between the first and second pairs of transducers so as to allow the probe assembly to operate in a variety of diameters of duct. This could be done before inserting the probe, or perhaps whilst it is inserted in the duct, or after insertion.
Additionally, the method may comprise the further step of comparing the transit times for signals propagating along each of the two paths in order to measure the swirl of fluid in the duct.
The method may comprise the extra step of measuring the duct diameter by monitoring transit time and/or signal amplitude (which is dependent on duct internal diameter and speed of sound in the fluid) in order to obtain an accurate measure of the duct diameter, and/or transducer alignment. This can enable the accurate alignment of the probe, or could be used to produce a signal which would provide feedback to the operator about probe alignment.
In accordance with a third aspect of the invention, we provide a method of checking a fixed flow meter by using an ultrasonic insertion flow meter in accordance with the first aspect of the present invention. The method may therefore comprise obtaining flow rate measurements by inserting the flow meter into a duct and comparing these measurements to the measurements obtained by the fixed flow rate meter in order to check the calibration or correct operation of the fixed flow meter.